For the first time, scientists at The University of Manchester’s National Graphene Institute in the United Kingdom have successfully created artificial channels that measure only a single atom in size.

Image credit: The University of Manchester

Relatively similar to natural protein channels like aquaporins, the novel capillaries are sufficiently small to obstruct the flow of tiniest ions like Cl− and Na+ yet enable water to flow through easily. The structures, in addition to enhancing the fundamental insight into the molecular transport at the atomic scale, and particularly in biological systems, may prove useful in filtration and desalination technologies.

Obviously, it is impossible to make capillaries smaller than one atom in size. Our feat seemed nigh on impossible, even in hindsight, and it was difficult to imagine such tiny capillaries just a couple of years ago.

Sir Andre Geim, Study Team Lead and Professor, The University of Manchester

Aquaporins are naturally occurring protein channels that enable water to rapidly permeate through them, yet they tend to block hydrated ions larger than about 7 A in size because of electrostatic repulsion and steric (size) exclusion mechanisms. Scientists have been attempting to create artificial capillaries that function just like their natural equivalents; however, in spite of significant advances in the development of nanotubes and nanoscale pores, all such structures continue to be relatively larger than biological channels.

Now, Geim and coworkers have made channels that have a height of just 3.4 A, which is roughly half the size of the tiniest hydrated ions, like Cl− and K+, whose diameter is 6.6 A. These channels act extremely similar to protein channels in that they are sufficiently tiny to block these hydrated ions and, at the same time, are sufficiently large to enable water molecules, with a diameter of about 2.8 A, to flow through freely.

Most significantly, the structures could help in developing low-cost, high-flux filters for water desalination and associated technologies—representing a holy grail for investigators in the field.

Atomic-scale Lego

Reporting their findings in Science, the scientists created their structures with the help of a van der Waals assembly method, also called “atomic-scale Lego,” which was developed as a result of studies on graphene.

We cleave atomically flat nanocrystals just 50 and 200 nanometre in thickness from bulk graphite and then place strips of monolayer graphene onto the surface of these nanocrystals. These strips serve as spacers between the two crystals when a similar atomically-flat crystal is subsequently placed on top. The resulting trilayer assembly can be viewed as a pair of edge dislocations connected with a flat void in between. This space can accommodate only one atomic layer of water.

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According to Boya, employing the graphene monolayers as spacers is the first-ever approach and makes the novel channels to stand out from other earlier structures.

The Manchester researchers created their 2D capillaries that measured several microns in length and 130 nm in width. They arranged these capillaries on top of a silicon nitride membrane separating a pair of isolated containers to make sure that the channels were the sole pathway through which ions and water could flow.

To date, investigators had only been able to determine the flow of water via capillaries that had relatively thicker spacers (about 6.7 A high). Some of their molecular dynamics simulations denoted that tinier 2D cavities must collapse owing to van der Waals attraction existing between the opposite walls, but other calculations highlighted the fact that water molecules within the slits may, in fact, function as a support and inhibit even one-atom-high slits (only 3.4 A tall) from collapsing down. This is certainly what the Manchester researchers have currently discovered in their experiments.

Measuring water and ion flow

“We measured water permeation through our channels using a technique known as gravimetry,” stated Radha. “Here, we allow water in a small sealed container to evaporate exclusively through the capillaries and we then accurately measure (to microgram precision) how much weight the container loses over a period of several hours.”

To achieve this, the investigators claimed that they developed a huge number of channels (more than a hundred) in parallel to boost their measurements’ sensitivity. They also employed thicker top crystals to inhibit sagging, and clipped the capillaries’ top opening using plasma etching to eliminate any possible blockages by thin edges existing here.

In order to determine the flow of ions, the researchers applied an electric field to force the ions to travel through the capillaries and subsequently determined the ensuing currents.

If our capillaries were two atoms high, we found that small ions can move freely through them, just like what happens in bulk water. In contrast, no ions could pass through our ultimately-small one-atom-high channels. The exception was protons, which are known to move through water as true subatomic particles, rather than ions dressed up in relatively large hydration shells several angstroms in diameter. Our channels thus block all hydrated ions but allow protons to pass.

Dr Radha Boya, Study Co-Author, The University of Manchester

As these capillaries act in the same manner as protein channels, they will be useful for gaining a better understanding of the way ions and water perform on the molecular scale—as in angstrom-scale biological filters.

Our work (both present and previous) shows that atomically-confined water has very different properties from those of bulk water. For example, it becomes strongly layered, has a different structure, and exhibits radically dissimilar dielectric properties.

Sir Andre Geim, Study Team Lead and Professor, The University of Manchester

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